The present invention relates to a monitor circuit for a control rod drive system of the type utilized to control the energy output of a nuclear reactor, and in particular to a monitor circuit that is adapted to detect erroneous stator phase sequences in any of the power supplies in the control rod drive system.
The reactor core of a nuclear reactor contains an array of control rods which are typically divided into eight groups. The reactor core is comprised of four quadrants with the control rods from each group being symmetrically distributed about the core. Each group, therefore, may contain either four, eight, or twelve individual control rods with either one, two, or three control rods, respectively, being disposed in each quadrant.
The control rods in a fission-type nuclear reactor are utilized to regulate the fission process in the reactor core by absorbing neutrons which would otherwise be available for fission production. Therefore, it will be appreciated that the power output of the reactor is regulated by controlling the depth of insertion of the control rods into the reactor core. Each control rod in the reactor core has associated therewith a control rod drive mechanism (CRDM) which is operative to control the vertical position of the control rod. Typically, the CRDMs utilized in a nuclear reactor include six-phase d.c. stepper motors which are adapted to revolve in discrete increments as the various stator phase windings of the motor are sequentially energized. In order to maintain the symmetry of the reactor core, it is important that all the control rods in each group are moved in unison. Accordingly, all of the CRDMs in a group are generally powered by a single power supply. Thus, for a reactor core having eight control rod groups, there would be eight separate polyphase power supplies.
The stator phase windings for a six-phase d.c. stepper motor are evenly spaced 60.degree. apart and are designated .phi.A, .phi.B, .phi.C, .phi.AA, .phi.BB, and .phi.CC. Phase shifts are implemented in 30.degree. increments by sequentially energizing adjacent combinations of stator phase windings. An erroneous phase shift in the power supply can produce what is referred to as a "ratchet condition" if the CRDM is running at the time of the fault. The term "ratchet condition" refers to a condition wherein the clutch in the CRDM which couples the motor to the control rod is momentarily disengaged from the control rod by the fault sequence and then immediately re-engaged upon the succeeding phase shift in the power supply. Due to the weight of the control rod, the CRDM may be unable to halt the control rod from dropping into the reactor core despite the re-engagement of the clutch. Consequently, the jaws of the clutch will slip or "ratchet" over the threads of the control rod thereby potentially causing damage to the control rods.
Accordingly, it is the primary object of the present invention to provide a monitor circuit for a control rod drive mechanism that is adapted to detect abnormal phase sequences in the group power supply for the CRDMs.
In addition, it is another object of the present invention to provide a monitor circuit which is further adapted to produce a non-destructive trip condition upon the detection of a group power sequence fault which is effective to de-energize the entire power supply for the affected group in the event the fault sequence occurs when the CRDMs in the group are running.
Furthermore, it is an object of the present invention to provide a monitor circuit that is adapted to detect an abnormal phase sequence in a group power supply and store the fault condition for identification.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :